222 PATTERNS AND PROBLEMS OF DEVELOPMENT 



dermal levels acropetally with increase in lithium effect, until in extreme 

 cases all the ectoderm had become entoderm and an organism consisting 

 only of entoderm and mesenchyme resulted. All gradations from the nor- 

 mal gastrula to these completely entodermized forms were found, accord- 

 ing to conditions; forms with partly evaginated, partly invaginated ento- 

 derm, entexogastrulae or exentogastrulae, were also described. Herbst was 

 at first inclined to regard exogastrulation as a more or less specific effect 

 of lithium; but he soon found that sodium butyrate and lack of mag- 

 nesium would also produce exogastrulation (Herbst, 1895, 1897). Since 

 Herbst's work exogastrulation has been produced by many investigators 

 in various species of sea urchins and several species of starfish not only 

 by lithium but by a great number of other agents, both chemical and 

 physical.^ Probably further experiment will add other agents to this list. 

 In the light of the data at hand it is evident that exogastrulation is not a 

 developmental modification determined by specific action dependent on 

 the chemical composition or specific physical effect of a particular agent 

 or agents and specific regional differences in egg or embryo. It is ex- 

 tremely improbable that all agents listed in the footnote act in the same 

 manner on particular regions of sea urchin, sand dollar, and starfish 

 embryos; but if they do not, the question at once arises whether exogastru- 

 lation may not be, like other modifications described in preceding sections, 

 a differential modification, depending on nonspecific alterations of a pri- 

 marily quantitative gradient pattern. « A recent study of differential dye 



s E.g., exogastrulation has been produced by NaCl, HgCU, CUSO4, KCN, NiCU, and by 

 various other salts and salt mixtures (MacArthur, 1924; Waterman, 1932, 1934, 1938), by 

 crowding and acidified sea water (MacArthur, 1924; Child), by high and low temperatures 

 (Driesch, 1893; Waterman, 1934), by neutral red (MacArthur, 1924), by methylene blue and 

 Janus green (Child), by auxin, glycogen, and HCIO3 (Motomura, 1934), by tobacco smoke 

 (Child), by lack of oxygen (Child), by X-rays, and occasionally a few exogastrulae by respira- 

 tory stimulants (Waterman, 1934, 1938). It has been observed by Runnstrom (1933) that 

 lithium and CO are additive in effect. In crowding lack of oxygen is probably the effective 

 factor rather than CO,, for the modifications occur when pH has not decreased below 7.3 or 

 7.4; but it is possible that an excreted toxic metabolite may be concerned. In acidified sea 

 water CO^ is probably the inhibiting factor. 



9 The writer's interest in the problem of exogastrulation, extending over many years, has 

 led, as opportunity offered, to a wide range of experiment, particularly with LiCl in concen- 

 trations ranging from the lowest effective to those completely inhibiting development and 

 killing rapidly and with exposure periods beginning at various developmental stages and rang- 

 ing from an hour or two to continuous, also some exposures to inhibiting agents before fertiliza- 

 tion. The following species have been used as material: echinoids: Arbacia piinctiilata, 

 Strongylocentrotiis franciscanns, S. purpuratus, Echinarachnius parma, and Detidraster excen- 

 tricus; asteroids: Asterias forbesii and Patiria niiniata. For the most recent general discussion 

 of echinoderm exogastrulation, a paper not included in the Bibliography, see Child, 1940, 

 "Lithium and echinoderm exogastrulation," Physiol. Zool., 13. 



